Not that I want to turn this into an extended rehash of that debate, but his main reason for doing it was to psychologically torture a foe he had already decisively defeated. His own future safety (a neutral motive, compatible with any alignment) was a consideration too, but I got the definite impression that this was, in V's then-current state of mind, a nice side-benefit; he was mainly out to make the dragon suffer. If that's not an Evil motive, I don't know what is.
I should add to this, that even if I agreed that his motive wasn't bad, in some cases a non-Evil motive isn't exculpatory. Otherwise we wouldn't look unkindly on those who recklessly endanger others - in most cases of this, the fact that they are endangering someone is the last thing on the perpetrator's mind, but far from excusing them, this is exactly the problem.
This was such a case - while V couldn't have predicted this specifically, she should have been alive to the possibility that in a group that large, there would be non-Evil individuals, especially if there were half-dragons involved. Indeed, one of the victims was pretty explicitly shown to most likely be a paladin - again, V had no direct way of knowing that specifically but it still illustrates that she should have taken the possibility into account. V knowingly engaged in an act she knew, or should have known, could have such tragic results as we see here.
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